The grafting of ethylenically unsaturated monomers on olefinic polymers, inclusive of the polyolefinic elastomers, in the presence of radicalic starters or initiators has been amply described in the literature. In this respect there may be cited: U.S. Pat. Nos. 3,177,269 and 4,010,223, British Pat. Nos. 852,042, 853,970, 905,043, 917,048, 972,229 and Belgian Pat. No. 558,004.
The grafting techniques used for the purpose differ from one another for the different physical state of the polymer to be grafted, wherefore we may have either processes in bulk, in solution or in suspension.
In the first case, the polymer and the unsaturated monomer are mixed together in the presence of free radical-generating compounds, at a temperature (in general comprised between 150.degree. and 180.degree. C.) which renders the material workable in extruders, mixers, etc.
However simple, this method is substantially little convenient because of the difficulty to control the operational conditions, especially when the olefinic polymer is rubbery.
The method in solution operates with the polymer dissolved in a solvent, usually an aromatic hydrocarbon, in the presence of the monomer to be grafted and of the other reactants, at temperatures greater than 70.degree. C., under stirring, for reaction times sufficient for getting high conversion rates. Such a method offers different advantages, amongst others the possibility of a thermal control; however, it is limited above all by the necessity to separate and recycle considerable quantities of solvent and non-solvent (coagulant) used for precipitating the grafted polymer from the solution.
The suspension-grafting method is realized by suspending, in general in water, the granulated polymer possibly blown with solvents, together with the monomer, the initiator or starter and a surfactant, and by then letting the mixture react at a temperature comprised between 60.degree. C. and 100.degree. C.
The suspension method offers the advantages of allowing to operate at high polymer concentrations and to separate the grafted polymer by means of simply filtering and centrifuging.
This method is not suited, however, for the grafting of monomers that are easily hydrolizeable in water, like maleic anhydride. Moreover, the grafting of the monomer takes place completely on the external surface of the polymeric granule and, thus, on the whole, with poor grafting yields on the total quantity of polymer used.